1. Field of the Invention
The present invention relates to a process for producing and obtaining 2,6-dialkylnaphthalene (DAN), in particular 2,6-dimethylnaphthylene  2,6-dimethylnaphthalene (2,6-DMN) from a mixture which contains at least one of dialkylnaphthalenes, monoalylnaphthalenes  monoalkylnaphthalenes or naphthalene.
2. Discussion of the Background
The compound 2,6-DMN is used as a precursor of 2,6-naphthalene dicarboxylic acid in the manufacture of high performance polyester resins such as polyethylene naphthalate polymer (PEN) or polybutyrene naphthalate polymer (PBN), because 2,6-DMN is easily oxidized to 2,6-naphthalene dicarboxylic acid compared with other precursors such as 2,6-diisopropylnaphthalene or 2-methyl-6-isobutyrylnaphthalenes. There have been many expected PEN's applications to film and bottle uses, such as long time recording type video film, Advanced Photo System, hot fill containers, refillable bottles and tire codes because of its good physical properties in strength, thermal resistance and gas barrier property. Expected PBN's main applications are for electronics, insulators and car parts. However, PEN and PBN have heretofore been too expensive to expand its market cleanly because of few effective processes for the 2,6-DMN commercialization.
There have been many proposals concerning the process for preparing the 2,6-DMN.
U.S. Pat. No. 4,795,847 (Weitkamp et al.) describes a process for the preparation of 2,6-dialkylnaphthalene by alkylating naphthalene or 2-alkyl-naphthalene with an alkylating agent in the presence of a zeolite (specially ZSM-5) as a catalyst.
U.S. Pat. No. 5,001,295 (Angevine et al) describes a process for preparing DMN by using 2-monomethylnaphthalene (MMN) and naphthalene as a feedstock and a synthetic zeolite (MCM-22) as a catalyst, and it shoes MCM-22 is more effective than ZSM-5 in alkylation of 2-MMN and naphthalene.
However these methods provide only unit operation (i.e batch) for alkylation of 2-MMN, which is an expensive feedstock and is not commercially available in a large amounts.
U.S. Pat. Nos. 4,990,717 (Sikkenga) and 5,073,670 (Sikkenga et al.) describes a multi-step process to produce 2,6-DMN from o-xylene and butadiene, which consists of;
1) preparation of 5-(o-tolyl)-pentene-2 (OTP) by alkenylation of o-xylene with butadiene in a presence of catalyst such as an alkali metal catalyst;
2) preparation of 1,5-dimethyltetralin (1,5-DMT) by cyclization of OTP in a presence of catalyst such as platinum and copper on an ultra stable zeolite catalyst;
3) preparation of 1,5-dimethylnaphthalene (1,5-DMN) by dehydrogenation of 1,5-DMT in a presence of catalyst such as platinum and rhenium and gamma alumina; and
4) preparation of DMN mixture which is rich in the desirable 2,6-DMN, 1,6-DMN and 1,5-DMN by isomerization of 1,5-DMN in a presence of catalyst such as a beta-zeolite catalyst.
If a 2,6-DMN separation from DMN mixture were combined with the above multi-steps, a complete process to produce purified 2,6-DMN could be provided.
As multiples steps makes a process plant complicate and in a high cost, it is hard to say that the prior art provides or a commercial process for an economical preparation of purified 2,6-DMN.
Furthermore, it is very difficult to separate 2,6-DMN from other isomers by conventional separation methods such as distillation and cooling crystallization because;
1) There are very small differences in boiling points of DMN isomers, especially the difference between 2,6-DMN and 2,7-DMN is only 0.3° C., where it is nearly impossible to separate 2,6-DMN by distillation.
2) The cooling of DMN isomer mixture solution of 2,6-DMN purification formes a precipitate of very fine 2,6-DMN crystals in suspension, where separation of the 2,6-DMN is extremely difficult.
Koide et al U.S. Pat. No. 4,992,619 reports a method of separating a methyl derivative of naphthalene from a mixture material in a high purity, by crystallization under a pressure.
Moritoki et al U.S. Pat. No. 4,784,766 reports a pressure crystallization apparatus.
Accordingly, method of commercially preparing dialkylnaphthalenes are sought.